About Me

No, not THAT Bob Hoover :-) (ie, Robert A. "Bob" Hoover from Tennessee and perhaps the best pilot in the history of flight.)
The problem is that all Roberts get Bob-ed at birth and there isn't much we can do about it. When posting something about aviation I generally use 'R.S.Hoover' to prevent confusion.

Sunday, May 20, 2007

Push-rod Tubes

It’s difficult to get excited about push-rod tubes, the eight little pipes that angle down from the heads to the crankcase. Nestled inside the lower plenum chambers, the push-rod tubes are out of sight and usually out of mind; properly installed, they’ll last the life of the engine. It came as quite a shock to learn they’re a part of the engine’s cooling system and a fairly sophisticated part too, for the role they play in regulating the engine’s oil temperature.

The first clue in understanding the Secret Life of Push-rod Tubes was learning that chromed push-rod tubes were part of the ‘high-latitude package,’ a kit of parts that re-configured the VW engine for operation in Arctic conditions. That’s alien stuff for most American VW mechanics, not because some parts of the States don’t have cold weather but because Volkswagen of America, the sales agent for VW products, didn’t bother to import such kits. Indeed, while Volkswagen produced more one-hundred thirty different types of vehicles, from track-laying snow-cats to farm tractors, VOA imported only half a dozen or so.

The cold weather kit consisted of chromed valve covers, push-rod tubes and sump-plate, three shutters that fit over the air-inlet on the blower housing, a different thermostat, split main bearings and an insulated blanket that wrapped around the engine’s sump. The purpose of the kit was to keep the heat in. The chromed parts were in direct contact with the engines oil and the shiny chrome surface reduced the thermal transparency of those parts by 90%, using the same principle found in shiny tea pots, chrome-plated percolators and the insides of a thermos-flask.

This puzzles a lot of folks who never took physics so let me explain that a shiny surface reflects heat. When you put chromed or polished valve covers on your engine, the heat in the oil sees that shiny surface from the inside and is bounced back into the engine. If you’ve got one of those infra-red thermometers you can prove this for yourself by simply putting a chromed valve cover on one bank and a stock valve cover on the other. Take it for a short run to warm things up then use the IR thermometer to measure the temperature of the valve covers. The shiny one will be as much as thirty degrees cooler than the black one.

Which is bad. Unless you live in Yellowknife :-) Because that’s thirty degrees of heat that is not being radiated out of the engine.

If you live in southern California and know your onions when it comes to VW engines, you’ll be running stock valve covers painted flat-black, which radiates even more heat than the stock gloss-black covers. Seeing shiny valve covers on a VW engine tells you all you need to know about the technological expertise of it’s owner :-)

As for how the push-rod tubes aid in the regulation of the engine’s temperature you need another shot of physics – the part where it tells you heat always runs ‘down-hill’ according to temperature. That means if you put something cool next to something hot, the cool thing will get hotter while the hot thing will get cooler. In designing the Volkswagen engine they took advantage of that fact by enclosing the push-rod tubes in the lower plenum chambers where they are bathed in air coming off the cylinders and heads. When the engine is first started the tubes are colder than the air coming off the engine, so the oil inside the tubes warms up. Once the engine reaches its normal operating temperature, which you want to happen as soon as possible, the oil inside the tubes will be hotter than the air coming off the engine so the oil will tend to cool down .

Like all car engines, the VW was designed for variable output, unlike an airplane engine that is designed for virtually a single level of output. Like most car engines the VW spends 98% of its service life producing an output approximately equal to a quarter of its maximum peak power. The only time it produces more is when accelerating or climbing hills and of the two, acceleration accounts for the majority. Not flat-out acceleration, as in seeing how fast you can go from a standing stop to your top speed, but relatively small accelerations, such as working your way through the gears, changing lanes and so forth. Those small bursts of power produce small bursts of excess waste-heat, most of which goes into the oil, which serves as a kind of temporary heat-sink.

Liquid-cooled engines are capable of dealing with significant excesses in the waste-heat department but air-cooled engines are not. They are designed for a relatively narrow operating window, which makes their use as a vehicle powerplant something of a challenge. Volkswagen addressed the problem by fitting an oil pump having more than eight times the output required for lubrication alone. The excess pumping capacity, in conjunction with an efficient heat exchanger and belt-driven blower allows the lubricant to serve as a coolant, which works quite well up to the engines maximum sustainable output of approximately 40bhp under Standard Day conditions. This system also works fairly well for occasional large increases in power of short duration, such as merging with traffic.

The push-rod tubes come into play for all of those small variations in output, such as working your way through the gears or when changing lanes.

The stock VW push-rod tube is the best option, being lighter and less expensive than the alternatives. Off-roaders or anyone who spends any time in the desert always carry a couple of spring-loaded push-rod tubes in case a stone get past the skid-plate but for flying Volkswagens the stock tubes do perfectly well. Assuming they are properly installed and that the engine is not allowed to over-heat.

Proper installation begins with a thin coat of flat-black paint to protect the bare steel and prevent it from rusting, since even a modest layer of rust serves as a remarkably good insulator, as any weldor can tell you. Flat-black paint because it has the best heat-transfer characteristic of any color. And a thin coat because all paints serve as insulators to some degree. But on the whole, a painted push-rod tube is a thousand times better than a rusty one.

Another aspect of proper installation has to do with the length of the push-rod tube, which is 7-9/16" (191mm to 192mm) for a stock engine. If your heads have been fly-cut of if your crankcase has been decked, you must subtract the depth of the cut from the 7.5625". In the same vein, anything that moves the heads farther from the center-line, such as barrel shims or head gaskets, must be added to the length.Adjusting the length is done by simply compressing the push-rod tube to make it shorter and extending it to make it longer. Unfortunately, if you do either of these things incorrectly you can harm the engine. To shorten a tube you drill a 7/8" hole in two pieces of wood at least 3/4" thick. 27/32" would be a better fit but most of you probably don’t have a set of wood-bits in 1/32" increments. (For the rest of the world use 22mm). Fit the drilled blocks over each end of the tube and compress them until the distance between the blocks is reduced to that required. You may rig a simple depth gauge if you wish. (A piece of welding rod works fine.)

The idea here is to compress the accordion-pleated ‘bellows’ portion of the push-rod without damaging the portion of the tube that projects beyond the bellows. The hazard here is two-fold: The projection portion of the tube is part of the sealing surface. Any bend or wrinkle usually results in a drip. Secondly, a bend in this area is liable to contact the push-rod itself.

But the most commonly required adjustment to a push-rod tube is to make it longer. Most of the manuals tell you to simply bend the bellows-section back & forth whilst pulling on the tube. This usually results in one accordion-fold being over-extended... which then gets squeezed back together when the heads are installed and quite often leads to the formation of a crack where the welded seam dips down into the fold of the bellows.

Why? Partly because of being over-extended but mostly because of what the bellows is designed to do, which is to maintain an oil-tight seal as the engine heats & cools. It is the thermal stress of those heat cycles that provides the energy for the over-extended portion to crack.

A better way to do it is to use a tubing cutter (!!) Set the blade so that it touches only the sides of the fold, press the push-rod tube into the cutter and simply swing it around the tube. It will force that fold of the bellows apart by some small amount. Repeat as needed to achieve the required length, dividing your work between both ends of the tube.This method works fine for one engine but it’s rather slow. If you’re building more than one engine at a time you’ll probably make yourself a tool similar to the one shown in the photos, which is nothing more than an old hacksaw blade or piece of steel strapping, stoned to a smoothly rounded edge and epoxied into a hardwood block. With the push-rod on a firm surface, press the tool against a fold and use the motion of the tool to roll the tube across the surface. The result is straighter tubes and more uniform spacing than is possible with the tubing cutter.

A key factor here is that the push-rod tube must never be longer than necessary. Some mechanics think they can prevent leaky push-rod tubes by starting out the things as much as an inch longer than required, then using the heads as a vise to achieve a crush-fit. The push-rod tubes are supposed to be straight. Crushing them into place almost always results in kinky looking tubes due to asymmetrical compression at one of the bellows sections.

Some big-bore strokers are as much as an inch wider than stock, meaning each push-rod must be extended by half an inch. The tool shown produces a consistent sixty-thou extension per pass. An extension of half an inch would take nine passes per tube, four on one end, five on the other. Since the tool goes into the valleys between the folds, and since there are twelve valleys on each bellows section, you would distribute the widening uniformly so as to equalize the stress.

The proper preparation and installation of your push-rod tubes is another example of the ‘unimportant details’ that spell the difference between a properly assembled engine and the other kind.